Tissue stimulators have gained wide acceptance in the field of medicine for the treatment of chronic, intractable pain. Generally, tissue stimulators include electrical circuits for generating electrical stimulation pulses, electrodes for attaching to the affected part of the body, and leads for conveying the stimulation pulses from the generating circuits to the electrodes. In some cases the entire tissue stimulator system is intended to be implanted within the body while in other cases the pulse generating circuitry is contained in a package external to the body. In this later case of transcutaneous stimulators, electrodes having a significant surface area are held in contact with the skin by adhesives or other means over the affected areas. Another type of stimulation which can be used either with external or implanted pulse generators uses leads extending to an implanted electrode, for example one implanted along the spinal cord. In any case, the application of the electrical stimulation pulses to body tissue produces the effect of relieving or masking the sensation of pain. In more sophisticated units, device controls or RF programming may be provided to turn on or off and/or adjust parameters of the output stimulation such as pulse width, pulse amplitude and repetition rate. Many tissue stimulator pulse generators also provide a burst or cycle mode wherein groups of stimulating pulses are provided at intervals, with delay intervals between the groups. An example of such a commercially available implantable device is the Medtronic Itrel II, Model 7424. This device is substantially described in U.S. Pat. No. 4,520,825, issued Jun. 4, 1985, to Thompson, et al. The Thompson '825 patent describes a circuit implementation of a cyclic gradual turn-on, or ramping of the output amplitude, of a programmable tissue stimulator. The implementation contains separate memory cells for programming the output amplitude and number of pulses at each increasing output level or "step."
In devices of this type, it is desirable to provide some means of control over the amplitude, or intensity, the frequency and the width of the stimulating pulses so that the patient using the device can adjust the device for maximum effectiveness. For example, if the amplitude is too low, them may be insufficient relief and if the amplitude is too high, there can be an unpleasant stinging or tingling sensation. The optimum stimulation parameters may change according to the time since implant and/or a variety of operating conditions including the length of time the stimulation has been on, patient postural changes, patient activity or the like.
Existing patient operated or "take home" programmers or transmitters for implantable tissue stimulators like the Medtronic Itrel II, Model 7424 or a transcutaneous tissue stimulator like the Medtronic Xtrel, Model 3470 have internal switches or controls that enable limiting of patient control over parameters and/or range of operation of selected parameters. Often times patients tamper with the limit setting switches thus causing subsequent lack of proper function. Additionally, during normal battery replacement, memory contents are lost requiring returning to the physician's office for proper resetting. An alternative method of programmer/transmitter parameter control limits would make use of a replaceable IC memory such as a Programmed Read Only Memory (PROM). However this method would severely limit the optimization of parameters for each patient as typical values would have to be preselected, fabricated and inventoried in each physician's office.